In the formation of a circuit pattern of a semiconductor device, a mask is used. In a process of manufacturing the mask, an electron beam lithography technique is used.
Electron beam lithography inherently provides a superior resolution, since a charged particle beam is used, more specifically an electron beam. This technology is also advantageous in that great depth of focus can be obtained, which enables dimensional variations to be reduced even when a large step feature is encountered. Moreover, an electron beam writing apparatus used for the electron beam lithography includes a system for data processing. Furthermore, this system also controls the drawing of circuit patterns, these circuit patterns can be complex and in a variety of designs.
Therefore the technology has been applied to the development of state-of-the-art devices typified by DRAM (dynamic random access memory), as well as to the production of some ASICs (application specific integrated circuits). Further effort is being made to develop an electron beam lithographic technique for writing a circuit pattern directly on a wafer with an electron beam.
The Patent Document 1 discloses a variable shape electron beam writing apparatus used for electron beam lithography.
The pattern writing data to be used in an electron beam writing apparatus is prepared using design data (CAD data) of a semiconductor integrated circuit, as one example. The data designed by a CAD system is processed in a predetermined fashion, and then divided into a plurality of stripes having the same width. The width of the stripes is a width, which can be deflected by a main deflector. Thereby the writing data of the entire chip has a hierarchical data structure including, the data of a plurality of strip-like stripes whose size corresponds to the size of the main deflection region, and the data of a plurality of sub-deflection regions, smaller in size than the main deflection region, in a single stripe.
When a pattern is written on a mask placed on a stage, while the stage is moved in a direction perpendicular to a width direction of the stripe, an electron beam is positioned in each sub deflection area. A predetermined position in the sub deflection area is irradiated with the electron beam.
When the mask placed on the stage in the electron beam writing apparatus is irradiated with the electron beam, reflected electrons are generated. The reflected electrons collide against an optical system, a detector, and so on provided in the electron beam writing apparatus and are charged up, whereby a new electric field is generated. Consequently, a direction of the electron beams deflected toward the mask is changed, and a drift in which a writing position deviates from a desired position occurs.
Although the drift is not due to only the above cause, in each case, it is required that a position of a reference mark on the stage is detected in the middle of writing to measure an amount of the drift, and the writing position is corrected so as to be located at a desired position. More specifically, the coordinates of the reference mark are obtained immediately before writing, and writing operation is then temporarily stopped during writing to obtain the coordinates of the reference mark again. Since a difference between the coordinates and the former coordinates is the drift amount, an electron beam writing position is corrected using the obtained value.
In conventional methods, the detection of the position of the reference mark in the middle of writing is performed at a predetermined time interval. More specifically, when writing of a stripe end is terminated in accordance with a previously set correction interval, the electron beam irradiation position is moved to the reference mark, and the reference mark is scanned with the electron beam to detect the position of the reference mark. In the drift correction, a value obtained by linear interpolation of a difference between the detected position and the last detected position is defined as a position variation before the start of the next detection. A permissible error after the drift correction is required to be satisfactorily smaller than positional accuracy of a mask pattern.
Recently, with high integration of a semiconductor integrated circuit, a circuit pattern is further miniaturized and complicated. Thus, higher positional accuracy of the pattern is required for the mask, and the error after the drift correction is required to be further reduced. However, when the drift correction is performed in accordance with the previously set correction interval, the number of times of measuring the drift amount is required to be increased to reduce a correction error, so that there is a problem that the entire writing time becomes longer.    Patent Literature 1: Japanese Laid-Open Patent Publication No. Hei 9-293670 (1997)    Patent Literature 2: Japanese Laid-Open Patent Publication No. Hei 7-142321 (1995)
The present invention has been made in view of the above problems. Namely, the present invention aims to provide a drift correction method, which detects a drift conventionally difficult to be detected and can improve correction accuracy, and a pattern writing data generation method suitable to be used with the drift correction method.
Other challenges and advantages of the present invention are apparent from the following description.